I see no mention of tamper-resistance/self-destruct features?
Power glitch detection, mechanisms to detect decapping/stripping, wire mesh shielding, protection against ultra-violet laser stimulation of transistors, ... are all important.
For those interested in further reading, Security Engineering[1] by Ross Anderson contains a section on chip security. Another paper[2] by Ross Anderson and Markus Kuhn (1996) provides additional background.